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What Would Make A Jet Engine Get High EGTs?  
User currently offlineSovietjet From Bulgaria, joined Mar 2003, 2608 posts, RR: 16
Posted (4 years 9 months 2 weeks 5 days 5 hours ago) and read 6423 times:
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Someone I know is experiencing some high EGTs on his L-39 jet. I was wondering what might cause this. I figure there is a variety of reasons. He has cleaned the turbine blades and that helped a little but still needs to get the temps down more. Anyone here have any experience or recommendations?

8 replies: All unread, jump to last
 
User currently offlineCrjfixer From United States of America, joined Mar 2008, 172 posts, RR: 0
Reply 1, posted (4 years 9 months 2 weeks 5 days 2 hours ago) and read 6399 times:

I have only worked on GE engines but the first thing i would check would be the probes themselves for false indications...More than likely it has 2 or more on each engine and you can isolate them and find a bad probe/rake.

Are all other indications normal? are the temps high at all power settings?


User currently offlineTristarSteve From Sweden, joined Nov 2005, 4007 posts, RR: 34
Reply 2, posted (4 years 9 months 2 weeks 4 days 23 hours ago) and read 6350 times:

Yes, I agree. Check out the EGT system first. An easy start is what does it read now. Should be the same as OAT!. Check connections and ballast resistors. They can be critical. On the RB211, even the stacking of the connectors on the lugs was important.
Then, if that is Ok, high EGT is due to high fuel flow. As an engine gets older the fuel flow and EGT will increase as the compressors and turbines get worn and have to work harder to do the job. I have seen over 50degC different EGT between two engines on the same aircraft, due to age. So you need to boroscope the engine to check the complete gas path for damage, not only blades and NGVs, but also the inside of the combuster for damage.. Take out the burners and check they are clean, they easily get clogged with carbon.

p.s., how do you clean the turbine blades? Is there only one stage?


User currently offlineCFMTurboFan From Canada, joined May 2007, 41 posts, RR: 0
Reply 3, posted (4 years 9 months 2 weeks 4 days 19 hours ago) and read 6256 times:

Yes Borescope the engine. Pay particular attention to the compressor section. if this is worn or dirty, then it will not be as efficient as it should be, which mean that the engine fuel system may be putting in more fuel to get the same performance out of the engine.

If the gas path, and all the blades look good, consider doing a compressor wash.


User currently offlineAvt007 From Canada, joined Jul 2000, 2132 posts, RR: 5
Reply 4, posted (4 years 9 months 2 weeks 4 days 11 hours ago) and read 6167 times:

Bleed air leaks will cause high EGT as well. Have any of the other engine parameters changed? Is he seeing higher fuel flow, or lower rpm, or is it temp alone?

User currently offlineSovietjet From Bulgaria, joined Mar 2003, 2608 posts, RR: 16
Reply 5, posted (4 years 9 months 2 weeks 3 days 7 hours ago) and read 5971 times:
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Thanks for the responses! We'll see what happens

User currently offlineAA737-823 From United States of America, joined Mar 2000, 5816 posts, RR: 11
Reply 6, posted (4 years 9 months 1 week 5 days 21 hours ago) and read 5727 times:

The EGT system is driven by thermocouple-type sensors. As such, things things RARELY, IF EVER LIE.
When they do, they lie low, not high.
And EGT indication that is above desired/expected is typically a true indication.

So, where do you go from here? Engine health. You say he's cleaned blades... good.
What about, as asked above, pneumatic leaks?
From there, you're really going to look at the FCU, but only if there is an associated error in rotational speed of the N2.
Not knowing which type of engine is on the L-39 (if he can't figure this problem out, he's more than welcome to donate the airplane to me... I promise I'll take good care of it), I can't get into specifics because I simply don't know them.

But, generally speaking, a high EGT indication is a TRUE indication, and is telling you that turbine blades, compressor blades, or FCU are having health problems or other issues.

And the FCU is a stretch, at that; the only thing an FCU (hydromechanical, which I'm assuming we're dealing with here, not digital) is sensing is RPM... not pressures, not temps, not what you had for breakfast, JUST RPM. So, unless there is also an RPM issue, stay out of the FCU.

He does realize that EGT rises continually as the engine ages... right? If it's gone past a limit, then... he's looking at a three-bolt fix, as we used to say on JT-8D's.


User currently offlineBoeingfixer From Canada, joined Jul 2005, 531 posts, RR: 0
Reply 7, posted (4 years 9 months 1 week 5 days 12 hours ago) and read 5680 times:



Quoting AA737-823 (Reply 6):
And the FCU is a stretch, at that; the only thing an FCU (hydromechanical, which I'm assuming we're dealing with here, not digital) is sensing is RPM... not pressures, not temps, not what you had for breakfast, JUST RPM. So, unless there is also an RPM issue, stay out of the FCU.

Actually most hydromechanical FCU's sense engine speed, compressor inlet pressure, compressor inlet temp, compressor discharge pressure and burner pressure with variations depending on the engine. It's slightly more complicated than sensing just engine speed.

For the OP, you should also check and make sure that the sensing lines going into the FCU are clear and open as well.

Cheers,

John



Cheers, John YYC
User currently offlineEx52tech From United States of America, joined Dec 2006, 559 posts, RR: 1
Reply 8, posted (4 years 9 months 1 week 4 days 8 hours ago) and read 5593 times:



Quoting Avt007 (Reply 4):
Bleed air leaks will cause high EGT as well. Have any of the other engine parameters changed? Is he seeing higher fuel flow, or lower rpm, or is it temp alone?

I would be looking for air leaks, but if the leaks are bad enough he can get a fire light. Wrap duct connections and flex bellows in tin foil and run the engine.

If you suspect the Fuel Control, or a sensing line leak like burner pressure, or discharge pressure, then check the acceleration of the engine to see if it is slower than recommended.

As already stated Borescope, will tell a lot. There may be some compressor blade tip clearance problems or turbine blade tip clearance issues. Compressor washes help, but usually not when the EGT is exceptionally high. Is this a high time engine? If at all possible try and borrow a fuel control from another operator and see what happens, just for process of elimination sake.

Quoting AA737-823 (Reply 6):
The EGT system is driven by thermocouple-type sensors. As such, things things RARELY, IF EVER LIE.
When they do, they lie low, not high.

That has always been my experience with the EGT system, they are very reliable.

High EGT is almost always F.O.D. or a wear issue.



"Saddest thing I ever witnessed....an airplane being scrapped"
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